Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults, accounting for most of 18,500 primary brain tumor cases each year in the US. Prognosis is dismal with a median survival of 12-15 mo, despite use of multimodality treatment. Novel therapeutic agents are urgently needed. Our group was the first to demonstrate that engineered measles virus (MV) strains have significant antitumor activity against gliomas. Their tumor specificity is due to abundant expression of the MV receptor CD46 in glioma cells. The virus upon entry in the tumor cells, causes membrane fusion with neighboring cells, syncytia formation and death. In addition, we have translated this approach into the first human clinical trial of a measles virus derivative producing human carcinoembryonic antigen, MV-CEA (CEA added to facilitate viral monitoring) in recurrent GBM patients. We now hypothesize that by introducing a therapeutic transgene and suppressing the innate immune response, we can further augment the antitumor activity of measles virotherapy in gliomas. We propose to accomplish this by testing the translational potential of three novel approaches in glioma treatment; a different measles virus strain, MV-NIS, which encodes the sodium iodine symporter (NIS) gene, thus allowing imaging of viral distribution in vivo; enhancing MV-NIS oncolysis, by exploiting NIS as therapeutic transgene with application of the beta and gamma emitter 131I (radiovirotherapy); and combining measles virus derivatives with cyclophosphamide, an agent that has been shown to suppress anti-viral innate and adaptive immunity, and increase viral proliferation in tumors. This grant proposal has therefore the following specific aims: 1) to test the efficacy of radiovirotherapy with MV-NIS in GBM lines and xenografts, compare it with MV-CEA and investigate the mechanism of antitumor activity; 2) to further increase the potency of measles virotherapy or radiovirotherapy by combining measles derivatives with cyclophosphamide (CPA), and optimize CPA's dose and schedule in this context; 3) to study viral biodistribution, interaction with the immune system and safety following intracranial administration, in two measles replication permissive animal models, Ifnarko CD46 Ge transgenic mice and Rhesus macaques.